This application is a National Stage of International Application No. PCT/GB97/02100, filed on Aug. 4, 1997, which claims priority to UK patent application No. 9617689.6 filed on Aug. 23, 1996, the contents of which are relied upon and hereby incorporated herein by reference.
This invention relates to an optical fibre grating and to an optical fibre transmission system using such optical fibre grating.
Long distance transmission at 10 Gbit/s over standard telecommunications fibre is of great interest because of the large base of such fibres already installed in the ground currently supporting low bit-rate systems.
The low loss of these already installed fibres, together with the ready availability of erbium doped fibre amplifiers (EDFAs), make the 1.55 xcexcm window an attractive wavelength region of operation. Unfortunately, however, the group velocity dispersion of these fibres is relatively large within this window, which severely limits the transmission distances achievable unless compensating techniques are employed.
Of the variety of methods which have been suggested thus far to solve this problem, linearly chirped fibre gratings as dispersion compensators are potentially very attractive, as they are compact, totally passive, and relatively simple to fabricate.
Progress in the development of fibre gratings has been rapid in the last few years, especially with the introduction of the phase mask technique, which provides a high degree of reproducibility in the gratings fabricated, as well as relaxing the tolerances on the fabrication set-up. In addition, longer fibre gratings are more readily realised with this approach than with the holographic technique. This is a crucial factor in dispersion compensation where the maximum compensatable distance is expected to scale directly with the grating length. Reports of experimental demonstrations of fibre compensation over standard fibre links, from 160 km, 220 km, to 270 km, were accomplished with gratings 4 cm to 12 cm long. With 10 cm long phase masks now commercially available, and even longer masks a likely prospect in the near future, fibre gratings capable of compensating over wide optical bandwidths and much longer distances should be feasible.
Recently, it has been experimentally demonstrated that dispersion compensation to 400 km of standard single mode fibre is possible, with a 3 dB power penalty, using a 10 cm long chirped fibre grating and an unchirped externally modulated transmitted. In separate experiments using a chirped externally modulated transmitter, transmission over 403 km with negligible penalty has been demonstrated and it has been shown that up to 537 km is possible with the use of two chirped gratings cascaded together.
This invention provides a chirped optical fibre grating formed by impressing a chirped substantially periodic refractive index variation on a polarisation-maintaining optical fibre.
This invention also provides an optical fibre transmission system comprising:
an optical transmitter;
a dispersive optical fibre link; and
a chirped optical fibre grating connected at or near to the input of the link to provide at least partial dispersion compensation to the light to be launched along the link, the grating being formed of polarisation-maintaining optical fibre and having a principal axis substantially aligned with a polarisation axis of light received from the optical transmitter.
The invention recognises that a dependence of system sensitivity on the input polarisation state to the grating can occur, due to polarisation mode dispersion (PMD) in linearly chirped dispersion compensating fibre gratings, and that this can give rise to a deterioration in the performance of optical transmission systems relying on such gratings for fibre dispersion compensation.
The invention addresses this problem by providing a polarisation-maintaining optical fibre grating, e.g. a chirped grating for dispersion compensation. The axes of the polarisation maintaining fibre of the grating can then be aligned with a polarisation axis of the light to be transmitted through a fibre transmission system.
The skilled man will understand that the term xe2x80x9cpolarisation maintaining fibrexe2x80x9d refers to optical fibre having an optically asymmetric cross section, and typically a birefringence of greater than about 10xe2x88x924.